OBJECTIVES: 1. Complete studies of allograft rejection in the mouse. 2. Continue to develop methods for the purification of human basophilic leukocytes from the blood of normal volunteers. 3. Nature of the inflammatory cell infiltrate and other fine structural alterations in primary cutaneous halo nevi. This entity may mimic malignant melanoma except that the lesions are characterized by spontaneous regression. Therefore, it provides an elegant model for study of the mechanisms by which humans reject tumors akin to melanoma.